Construction machinery may raise and lower a front work apparatus using a hydraulic cylinder to. For example, an engine power may be used to drive a hydraulic pump, and a hydraulic oil discharged from the hydraulic pump may be supplied to a boom cylinder through a main control valve to generate stoke of the boom cylinder, thereby raising a boom. On the other hand, when the boom is lowered, the hydraulic oil from the boom cylinder may be drained to a drain tank through the main control valve due to gravity of the front work apparatus. During the boom down operation, potential energy of the front work apparatus may not be effectively utilized. Accordingly, a new technique of regenerating the potential energy may have been developed.